Effect of thickness on the stability of transparent conducting impurity-doped ZnO thin films in a high humidity environment

The resistivity of transparent conducting Al- and Ga-doped ZnO (AZO and GZO) thin films prepared with a thickness in the range from 20 to 200 nm on glass substrates at a temperature below 200 degrees C was found to increase with exposure time when tested in a high humidity environment (air at 90% relative humidity and 60 degrees C). The resistivity stability (resistivity increase) was considerably affected by the thin film thickness. In particular, thin films with a thickness below about 50 nm were very unstable. The increase in resistivity is interpreted as carrier transport being dominated by grain boundary scattering resulting from the trapping of free electrons due to oxygen adsorption on the grain boundary surface.